To offer a digital television service, a television operator or broadcaster transmits video contents over satellite, cable or radio broadcast networks, over IP broadband networks or again over mobile networks to several clients at a time according to the principle of broadcast or multicast diffusion. Television over IP (or IPTV) has developed since the explosion in end user very high bitrate access offers. The broadcasting of video requires a high bitrate on the network. On open wired (Open Internet type) or managed (by a Free type access provider) networks, the bandwidth of the access point of the client conditions the eligibility of said client for the IPTV services and also conditions the coding quality of video contents, for example high definition (HDTV). Thus depending on his bandwidth, the client receive no television channel or no Video On Demand service at all, receive TV programmes in standard quality, or receive different contents in high definition. The bandwidth of the client depends on technical characteristics of the access network and on the congestion of the network. For broadcasting over mobile networks, such as 3G telephony, the problem of limitations of the bandwidth is also present, the constraint in term of bandwidth even being higher.
Different solutions are proposed to increase the eligibility of individuals for IPTV or VOD services and to improve the quality of broadcast contents. A first series of solutions exists based on image processing, such as SD-HD upscaling or digital zoom including filtering. However these solutions offer a mediocre result compared to the original HD content. In the networks domain, very high bitrate accesses are being developed. In the compression domain, the use of standards such as H.264/MPEG-4 AVC (Advanced Video Coding) or H.264/MPEG-4 SVC (Scalable Video Coding) enables the improvement of the compression rate of broadcast contents. SVC also enables the transport to be split into different streams adapted to the different capacities of broadcast networks thus offering a video quality that can be adapted to the bandwidth of receivers. In fact, according to the “scalability” model, different SVC streams correspond to different successive levels of quality of a same content (hierarchically dependent and coded incrementally) and the AVC base layer can be decoded without the enhancement layers. However, the different streams are transmitted simultaneously on the network. Thus, a client can benefit from an IPTV service in standard definition if his bandwidth is sufficient to receive a base layer in real time but can not benefit from an IP-HDTV service if his bandwidth is insufficient to receive the enhancement layers in real time.
In the context of a unicast transmission on a wideband network, the document WO 03/04783, describes a method for broadcasting of music in which the data streams are divided and are not transmitted simultaneously. A first stream is previously transmitted then it is stored in memory. A second stream is transmitted in real time, it is combined with the first stream in a way to enable the reproduction of a piece of music in real time. This solution enables the real time constraints on the complete stream to be overcome and enables the bandwidth in real time resources to be optimised. Still in the domain of unicast transmissions, the document U.S. Pat. No. 6,496,980 describes a method for unicast transmission with two non-simultaneous streams. The base layer is received in real time and offers a standard quality. Upon request by the user, the enhancement layers are downloaded thus offering the user the advantage of a new reproduction of the video content with an improved quality.
These solutions do not address the technical problem of eligibility for a television broadcast service on a wideband network for which the bandwidth of the access point does not enable the reception in real time of video streams of a quality sufficient for its reproduction. These solutions also do not address the technical problem of the broadcasting of a HD television service in real time in particular on a “broadcast” network or on a wideband network for which the bandwidth of the access point is insufficient.